1. Field of the Invention
This invention relates to a novel photopolymerizable laminate. More particularly, the present invention pertains to a photopolymerizable laminate which is useful for preparation of a printed circuit board.
2. Description of the Prior Art
As photoresist materials for preparation of printed circuit boards, photopolymerizable elements, so-called dry film resists, which comprise a photopolymerizable layer and a support layer have been known. The dry film resists are usually prepared by laminating a photopolymerizable material on a flexible support film, and optionally covering the surface of the photopolymerizable material with a protective film. When preparing a printed circuit board by use of the dry film resist (hereinafter referred to as "DFR"), first after peel-off of the protective layer, the photopolymerizable layer is laminated on a substrate for forming permanent images, for example, a copper-clad laminate and then subjected to imagewise exposure, followed by peel-off of the support layer, if desired. Subsequently, by using a developing liquid, the support layer left ro remain as desired and the unexposed portion of the photopolymerizable layer are removed by dissolution of dispersion to form a resist image on the surface of the substrate. Then, a desired printed circuit board can be obtained by two ways. One way consists of etching the portion of the substrate where the resist is not formed and stripping the resist. The other way consists of plating the portion of the substrate where the resist is not formed, stripping the resist, and etching the substrate where the resist has been stripped.
Whereas, recently, among the printed circuit boards prepared by use of DFR, increased is the printed circuit boards with electrical connection between one surface and the other surface of the boards through an electroconductive circuit-forming material layer, for example, a thin copper layer, provided on the inner surfaces of thru-holes. As the method for preparation of printed circuit of this kind, the copper thru-hole methods and the solder thru-hole methods are generally known, and the tenting method is becoming popular among the copper thru-hole methods. In the tenting method, simultaneously with formation of wiring images by exposure, both openings of the thru-hole are completely sealed with the photo-hardened layer. During etching of the copper thin layer, etc., in order to form a wiring, said photohardened layer protects the electroconductive layer on the inner surface of the thru-hole against etching solution and therefore it is possible to obtain a printed circuit in which the both surface are conductive to each other through the thru-hole. The electroconductive circuit-forming material layer on the peripheral surface of the openings of the thru-hole may have a width which is very limited due to the restriction in circuit designing. Accordingly, in the above-mentioned tenting method, the photopolymerized layer sealing the openings on both sides of the thru-hole is supported on a band portion with a very narrow width which is called land. The exposed photopolymerized layer sealing the openings will be showered with developing solution and etching solution and therefore may frequently suffer from breaking of the photopolymerized layer or peeling off from the substrate. For this reason, the photopolymerizable layer to be used for the tenting method is required to have good mechanical strength, good adhesiveness to the substrate and chemical resistance after exposure.
In this context, the photopolymerizable layer of DFR widely used at present time comprises (1) an unsaturated compound having at least one ethylenic group capable of photopolymerization as a monomer, (2) a thermoplastic organic polymer as a binder, (3) a photopolymerization initiator and (4) other additives (see U.S. Pat. Nos. 3,622,334 and 4,320,189).
The above-mentioned monomer may include trimethylolpropane triacrylate and pentaerythritol triacrylate having two or more ethylenic double bonds. As another example, there is also shown triethyleneglycol diacrylate having ether linkage. On the other hand, as the binder, those compatible with monomer are chosen, as exemplified by polymethyl methacrylate, copolymers of methacrylic acid ester, acrylic acid ester or styrene.
However, the mechanical strength or the adhesiveness with a substrate of the photopolymerized layer is influenced by the monomer, the binder and their ratios employed, particularly greatly by the monomer. The mechanical strength, chemical resistance and adhesiveness with the substrate of the photopolymerizable layer employing the monomer as mentioned above are not necessarily satisfactory, when the photopolymerizable layer is thin. Accordingly, when using the tenting method, the photopolymerizable layer must be made thick.
On the other hand, demand for miniaturization of electronic parts and elements is increasingly strong, and correspondingly the printed circuit board with higher density or finer lines is strongly desired.
For formation of such a high density printed circuit, the DFR employed must have a high resolution, but the resolution is lowered by increasing the thickness of the photopolymerizable layer. Accordingly, when image formation is effected by applying the DFR comprising the photopolymerizable layer as described above for the tenting method, it is difficult to obtain a circuit image which is very precise and of high density.
On the other hand, according to the solder thru-hole method widely used also at the present time, the electroconductive circuit-constituting material layer, for example, copper thin layer, is covered with a protective metal such as solder before etching of the unnecessary portions on the surface of the substrate for forming permanent images. In the solder thru-hole method, the substrate covered with a patterned photopolymerized layer is subjected to cleaning for copper plating such as defatting by alkali washing, defatting by washing with an aqueous surfactant solution or electrolytic polishing, and then to electroless copper plating and further to copper plating and solder plating. Accordingly, also in the case of preparing a high density printed circuit, a line distance of the circuit will become narrower.
Thus, when the line distance becomes narrower, penetration of a plating solution or a cleaning solution into the resist or between the resist and the substrate and small float of the resist during cleaning before plating, copper plating and solder plating treatment pose great problems, and therefore DFR is required to have excellent resistance to cleaning reagents, copper plating and solder plating. The copper plating resistance, solder plating resistance and resistance to cleaning reagents of the photopolymerized layer are also influenced by the monomer and the binder employed for the photopolymerizable layer, particularly predominantly by the monomer and its amount. The photopolymerizable layer using as the monomer an acrylic acid ester or a methacrylic acid ester having only an ester bond or an ester bond and an ether bond such as trimethylolpropane triacrylate or polyethylene glycol diacrylate is insufficient particularly in copper plating resistance, solder plating resistance and resistance to pre-treatment chemicals.
In order to compensate for such drawbacks, for example, it has been proposed to add a heterocyclic compound to the photopolymerizable layer (see U.S. Pat. No. 3,622,334 and Japanese Laid-open Patent Publication No. 192946/1982). However, such composition has a disadvantage that the additive may sometimes be precipitated during storage of DFR for a long term.
G.B. Pat. No. 1379228 discloses a photopolymerizable copying material using a monomer represented by the formula (A') shown below: ##STR2## wherein
X' is a saturated hydrocarbon radical with 2 to 12 carbon atoms or an arylene radical with 6 to 10 carbon atoms,
R.sub.1 " and R.sub.1 ', which are identical or different, are each alkyl with 1 to 3 carbon atoms, R.sub.4 ' or CH.sub.2 R.sub.4 ',
R.sub.2 ", R.sub.2 ', R.sub.3 " and R.sub.3 ' are identical or different and are each H, CH.sub.3 or CH.sub.2 R.sub.4 ',
R.sub.4 ' is O--CO--CR.sub.5 '.sub.5 Z.dbd.CH.sub.2,
R.sub.5 ' is H or CH.sub.3, and
n.sub.1 and m.sub.1 are identical or different numbers from 0 to 20,
R.sub.4 ' being present in at least one of the radicals R.sub.1 ", R.sub.2 " and R.sub.3 " and in at least one of the radicals R.sub.1 ', R.sub.2 ' and R.sub.3 '.
However, the monomer represented by the formula (A') is small in solubility in a conventional developer for manufacturing of printed circuit boards using DFR, such as 1,1,1-trifluoroethane, trichloroethylene, etc. This means that minute amount of monomer will remain on the substrate, even when attempted to remove the unexposed portion by developing, which poses the problem that subsequent steps of plating, etching, etc. will not proceed uniformly.
As the improved method for preparation of a printed circuit board using DFR, it has been proposed to use a photopolymerizable laminate whose support layer can be dissolved or dispersed in a liquid for developing of the photopolymerizable layer and, after exposure for image formation, to remove by developing the flexible support layer and the unexposed portion of the photopolymerizable layer as such, without peeling off the flexible support layer from the photopolymerizable layer, thereby forming a resist image on a substrate for forming permanent images (U.S. Pat. Nos. 4,211,560, 4,301,230 and 4,360,582). This method has the advantage of simplifying the steps in preparation of printed circuit boards.
However, in the case when using as the monomer in the photopolymerizable layer in this method, for example, an acrylic acid ester or a methacrylic acid ester having only an ester bond or an ester bond and an ether bond such as trimethylolpropane triacrylate or polyethylene glycol diacrylate, if lamination is performed at a high temperature while applying an excessive tension on DFR, there may be generated cracks on the flexible support layer laminated on the substrate for forming permanent images.